1. Field of the Invention
The present invention relates to an ink jet head which ejects ink droplets so as to make the ink droplets adhere onto recording paper only when recording is demanded; an ink jet printer thereof; and a method for driving the ink jet head. In particular, the present invention relates to the prevention of a failure or abnormality in ink ejection.
2. Description of the Related Art
Generally, an ink jet head has pressure build-up chambers for applying pressure to ink so as to eject ink droplets. Then, one end of each pressure build-up chamber communicates with an ink tank through an ink supply channel while the other end of the pressure build-up chamber is provided with an ink nozzle for ejecting an ink droplet. In addition, a bottom portion of the pressure build-up chamber is formed to be deformable and used as a diaphragm. This diaphragm is elastically displaced by electromechanically converting means so as to generate pressure for ejecting an ink droplet from the ink nozzle.
A printer using such an ink jet head has excellent features such as low noise, low power consumption, and so on, and it has come into wide use as an output unit for an information processor. On the other hand, in the ink jet head, menisci in the ink nozzles are pushed out in unstable forms by remaining vibration generated in the pressure build-up chambers. As a result, unnecessary ink droplets constructing no printing may be ejected immediately after necessary ink droplets are ejected. The ejection speed of the unnecessary ink droplets constructing no printing is so low that they adhere to nozzle surfaces and cause a phenomenon such as ink nozzle clogging or dot missing. Thus, the reliability on printing is lowered.
Further, when the printer is left for a long time in the state where the ink jet head is not driven, water, or the like, which is a solvent of ink, evaporates through the ink nozzles. As a result, the viscosity of ink in the ink nozzles increases so that the ink nozzles are clogged. Moreover, with the increase of the ink viscosity, the refil speed of the ink nozzles with ink becomes so low that the refill quantity cannot follow the ink ejection quantity. As a result, bubbles are mixed into ink so that the ink jet head is in a non-ejection state where no ink droplet is ejected. Thus, the reliability on printing is lowered in the same manner as mentioned above.
In the background art, for the former where a failure in ejection is caused by ink adhesion to nozzle surfaces, the nozzle surfaces are rubbed with a wiper (wiped) before the beginning of printing or during a rest period of printing, so that the nozzle surfaces are prevented from wetting due to the adhesion of unnecessary ink droplets to the nozzle surfaces. Further, the publication JP-A-4-369542 discloses a technique in which a second voltage different from a first voltage for ejecting ink droplets is applied to electrostrictive members so as to separate ejected ink droplets and reduce the ejection of unnecessary ink droplets.
On the other hand, for the latter where a failure in ejection is caused by ink nozzle clogging and bubbles in ink, the operation of ejecting several shots of ink droplets, that is, so-called pre-ejection is performed before the beginning of printing or during a rest period of printing. Further, the publication of JP-A-9-30007 proposes a method in which a pulse with electric power at the level at which no-ink droplet is ejected from ink nozzles is applied to electrostrictive members so as to micro-vibrate menisci in order to prevent the ink nozzles from being filmed with ink.
However, the above-mentioned background-art techniques have problems as follows.
1. In the wiping operation, there was a problem that printing time was elongated because the ink jet head had to be moved to shelter at a place other than a print area at any time when wiping was performed. In addition, there was a problem that water-repellant coatings on the nozzle surfaces were deteriorated by the repeated wiping of the nozzle surfaces.
2. In the case where a voltage was applied to the electrostrictive members in order to separate ink droplets, characteristic differences between the electrostrictive members might make it impossible to separate the ink droplet well and might eject even unnecessary ink droplets. Thus, there was a problem that it was difficult to attain stable ejection and separation of ink droplets.
3. In the pre-ejection operation, there was a problem that ink irrelevant to printing was markedly consumed so that the life of the ink tank was shortened. In addition, there was a problem that printing time was elongated because the ink jet head had to be moved to shelter at a place other than a print area at any time when pre-ejection was performed.
4. In regard to the driving method to apply such a low pulse voltage as to eject no ink droplets, if this method was applied to an ink jet head using electrostatic driving actuators, it was difficult to set a driving condition on which menisci were vibrated without ejecting any ink. Accordingly, there was a problem that ink droplets were ejected, or enough vibrations of the menisci to avoid a failure in ink ejection were not obtained. In addition, it was necessary to give driving signals to driving elements for all the ink nozzles respectively. Accordingly, there was a problem that driving control was complicated, etc.
It is an object of the present invention to provide an ink jet head that eliminates or reduces printing trouble caused by a failure or abnormality in ink ejection; an ink jet printer using such an ink jet head; and a method for driving such an ink jet head.
(1) An ink jet head according to the present invention comprises a plurality of ink nozzles for ejecting ink, a plurality of ink chambers respectively communicating with a corresponding one of the ink nozzles, ink supply channels respectively supplying ink to a corresponding one of the ink chambers, elastically displaceable diaphragms respectively formed in a wall of a corresponding one of the ink chambers, and opposed electrodes oppositely arranged to the diaphragms through a gaps, to eject ink droplets from the ink nozzles by performing electric charge/discharge between the opposed electrodes and the diaphragms; wherein each of the opposed electrodes comprises a main electrode that can perform electric charge/discharge between it and a corresponding one of the diaphragms independently of the other main electrodes, and a sub-electrode that is electrically connected with the sub-electrodes for the other diaphragms.
In the present invention, the electrodes are driven in a desired combination (driving voltages are applied between the opposed electrodes and the corresponding diaphragm so as to perform electric charge/discharge therebetween), so that the quantity of ink ejected from an ink nozzle (density) can be adjusted in multiple stages. In addition, since each sub-electrode is electrically connected with the other sub-electrodes formed for the other diaphragms, a process for vibrating ink in the ink nozzles can be performed in common for the respective ink chambers. Thus, the control of such a process becomes easy.
(2) In the ink jet head according to the present invention as stated in paragraph (1), each main electrode is electrically charged and discharged selectively in accordance with a printing pattern, and a sub-electrode formed on the ink nozzle side is electrically connected with sub-electrodes formed for the other diaphragms. In the present invention, main electrodes are driven selectively in accordance with a printing pattern so that a process of printing is performed. In addition, sub-electrodes are driven appropriately so that ink in the ink nozzles can be vibrated or the effect of separating ejected ink droplets from the ink nozzles can be enhanced. That is, auxiliary electric charge is performed between a sub-electrode and diaphragm so that parts of the diaphragm are bent toward the sub-electrode. Thus, menisci or ink of the ink nozzles can be vibrated without ejecting unnecessary ink droplets. As a result, the menisci can be prevented from being filmed with ink, without ejecting ink droplets. In addition, ink in ink channels is diffused so that the viscosity of the ink can be prevented from increasing due to the evaporation of the solvent of the ink. Further, if sub-electrodes are driven before ink droplets are ejected, troubles in printing caused by a failure or abnormality in ink ejection can be prevented without consuming ink playing no part in printing, even after no ink droplets has been ejected for a certain time because of no operation of the ink nozzles.
(3) In the ink jet head according to the present invention as stated in paragraph (2), a first gap between the main electrodes and the diaphragms is made different from a second gap between the sub-electrodes and the diaphragms. According to the present invention, for example, auxiliary electric charge is performed between a sub-electrode and diaphragm so that a part of the diaphragm is bent toward the sub-electrode. As a result, the timing when a tail portion of a discharged ink column is separated from ink in the ink nozzle can be hastened so that the effect of separating an ink droplet from the ink nozzle can be further enhanced.
(4) In the ink jet head according to the present invention as stated in paragraph (3), the first gap is set to be larger than the second gap. In the present invention, for example, when a driving voltage equivalent to the driving voltage for a main operation (ink ejection) is applied for an auxiliary operation, a Coulomb force produced in the auxiliary operation is larger than Coulomb force produced in the main operation so that the bending speed of the diaphragm in the auxiliary operation becomes higher than that in the main operation. As a result, the operation that a meniscus in the ink nozzle is drawn into the ink chamber is hastened so that the tail portion of the ejected ink column can be separated more surely in the auxiliary operation. Thus, it is possible to form ink droplets stably.
(5) In the ink jet head according to the present invention as stated in paragraph (2), the main electrodes are provided correspondingly to the diaphragms, and each sub-electrode includes a first sub-electrode provided in common for the plurality of diaphragms so as to face the diaphragms on the ink nozzle side, and one or a plurality of second sub-electrodes provided in common for a plurality of the diaphragms so as to be disposed between the main electrodes and the first sub-electrode.
In the present invention, the sub-electrodes are divided in series so that the electrostatic capacity thereof is reduced. Thus, the time constant of the sub-electrodes are prevented from increasing, so that the difference between the time constant of a circuit associated with a main electrode and the time constant of a circuit associated with a sub-electrode is reduced. As a result, proper control timing can be obtained easily for controlling both the electrodes. In addition, the operation delay among auxiliary actuators formed by the sub-electrode is also reduced so that the proper operations of the main and sub-electrodes can be obtained.
For example, in the case where the main electrode and the sub-electrode are driven simultaneously so that control is made for increasing the quantity of ink to be ejected in comparison with the case where only the main electrode is driven (that is, control is made for adjusting the printing density in multiple stages), or in the case where the sub-electrode is driven at a predetermined time after the main electrode was driven so that control is made for cutting the tail portion (rear end) of the ejected ink column to avoid production of a surplus ink droplet, proper timings of the control can be obtained, since the difference between the time constants of the respective circuits associated with the main electrode and the sub-electrode is small. As a result, precise printing control can be performed. Incidentally, the concept of the time constants of the respective circuits in the present invention will be described in detail later in Embodiment 4. In addition, according to the present invention, the sub-electrode is constituted by a plurality of electrodes so that the ink ejection quantity (density) can be adjusted in more multiple stages. In addition, the sub-electrode is formed in common for a plurality of diaphragms so that increase of the number of wires connecting the electrodes, which is involved by increase of the number of ink nozzles, can be avoided. Thus, increase in size of the ink jet head can be avoided.
(6) In the ink jet head according to the present invention as stated in paragraph (2), each of the main electrodes and sub-electrodes includes an opposed portion formed of ITO and oppositely disposed to the diaphragm, and a lead portion electrically connected with the opposed portion, wherein at least the lead portion of the sub-electrode is formed of metal. In the present invention, at least the lead portion of the sub-electrode is composed of metal so that the time constant of the circuit associated with the sub-electrode is reduced. As a result, the difference between the time constant of the circuit associated with the sub-electrode and the time constant of the circuit associated with the main electrode is reduced.
(7) In the ink jet head according to the present invention as stated in paragraph (6), the metal is composed of gold formed on chromium or titanium. The metal is attached to the substrate stably, so that it withstands long-term use without fear of peeling off.
(8) In the ink jet head according to the present invention as stated in paragraph (2), the diaphragms are formed as a common electrode, and a time constant of a circuit constituted by each electrode of the opposed electrodes and the common electrode is much smaller than a natural vibration period of corresponding one of the ink channels. Accordingly, the difference between the time constants of the respective circuits is also reduced, so that proper control timing can be obtained easily. In addition, an operation delay caused between auxiliary actuators formed by the sub-electrodes is also reduced so that proper operations of the main electrodes and sub-electrodes can be assured.
(9) In the ink jet head according to the present invention as stated in paragraph (2), the main electrodes are provided correspondingly to the diaphragms while a sub-electrode is provided in common for a predetermined number of the diaphragms so as to face the diaphragms on the ink nozzle side, wherein a plurality of umits each having a predetermined number of main electrodes and a sub-electrode are disposed. Since the sub-electrode is divided in parallel so that the respective capacities of the divisional electrodes are reduced, the time constant of the circuit associated with the sub-electrode is prevented from increasing. As a result, the difference between the time constant of the circuit associated with the main electrode and the time constant of the circuit associated with the sub-electrode is reduced. In addition, a sub-electrode is formed in common for a plurality of diaphragms so that, even if the number of ink nozzles increases, the number of wires connected to the sub-electrodes can be prevented from increasing in accordance therewith. Thus, the above-mentioned operations can be attained without increasing the number of wires in the ink jet head or without increasing the number of wires connecting a control circuit with the ink jet head.
(10) In the ink jet head according to the present invention as stated in paragraph (9), every adjacent two of the units are disposed to be symmetrical with respect to a boundary line between the units. Since every two units are arranged in parallel and symmetrically in such a manner, no sub-electrode lies between the main electrode groups of the two units. Therefore, when the ink jet head is manufactured, pattern groups of the main electrodes with one and the same pitch may be produced. Thus, the ink jet head is manufactured easily.
(11) According to the present invention, there is provided an ink jet printer comprising an ink jet head which includes a plurality of ink nozzles for ejecting ink, a plurality of ink chambers communicating with the ink nozzles respectively, ink supply channels for supplying ink to the ink chambers respectively, elastically displaceable diaphragms formed in circumferential walls constituting the ink chambers respectively, and opposed electrodes oppositely arranged to the diaphragms through a gap respectively, to eject ink droplets from the ink nozzles by performing electric charge/discharge between the opposed electrodes and the diaphragms; wherein each of the opposed electrodes are constituted by a plurality of electrodes each of which can perform electric charge/discharge to corresponding one of the diaphragms independently of the other electrodes, and at least one of the plurality of electrodes is electrically connected with the electrodes formed for the other diaphragms. In the present invention, the plurality of electrodes in an opposed electrode are driven in a desired combination, so that the quantity of ink ejected from an ink nozzle (density) can be adjusted in multiple stages. In addition, since at least one of the plurality of electrodes is electrically connected with the other electrodes formed for the other diaphragms, for example, a process for vibrating ink in the ink nozzles can be performed in common for the respective ink chambers. Thus, the control of such a process becomes easy.
(12) In the ink jet printer according to the present invention as stated in paragraph (11), an opposed electrode includes a main electrode to be electrically charged and discharged selectively in accordance with a printing pattern, and a sub-electrode formed on the ink nozzle side and electrically connected with sub-electrodes formed for the other diaphragms. In the present invention, main electrodes are driven selectively in accordance with a printing pattern so that a process of printing is performed. In addition, sub-electrodes are driven appropriately so that ink in the ink nozzles can be vibrated or the effect of separating ejected ink droplets from the ink nozzles can be enhanced.
(13) The ink jet printer according to the present invention as stated in paragraph (12) comprises a main electrode driving circuit for electrically charging/discharging the main electrodes and the diaphragms so that ink droplets are discharged from the ink nozzles; and a sub-electrode driving circuit for electrically charging/discharging the sub-electrodes and the diaphragms in a predetermined period or at a desired time so that ink in the ink nozzles is vibrated. In the present invention, the main electrodes are driven by the main electrode driving circuit so as to eject ink droplets, and the sub-electrodes are driven by the sub-electrode driving circuit so as to vibrate ink in the ink nozzles.
(14) The ink jet printer according to the present invention as stated in paragraph (12) comprises: a main electrode driving circuit for electrically charging/discharging the main electrodes and the diaphragms so that ink droplets are ejected from the ink nozzles; and a sub-electrode driving circuit for electrically charging/discharging the sub-electrodes and the diaphragms at a desired time after electrically discharging the main electrodes, so that ink ejected from the ink nozzles are separated from ink remaining in the ink chambers. In the present invention, the main electrodes are driven by the main electrode driving circuit so as to eject ink droplets, and the sub-electrodes are driven by the sub-electrode driving circuit so as to separate ink ejected from the ink nozzles, from ink remaining in the ink chambers.
(15) According to the present invention, there is provided a method for driving an ink jet head which includes a plurality of ink nozzles for ejecting ink, a plurality of ink chambers communicating with the ink nozzles respectively, ink supply channels for supplying ink to the respective ink chambers, elastically displaceable diaphragms formed in circumferential walls constituting the ink chambers respectively, and opposed electrodes oppositely arranged to the diaphragms through a gap respectively, to eject droplets from the ink nozzles by performing electric charge/discharge between the opposed electrodes and the diaphragms; wherein each of the opposed electrodes is constituted by a plurality of electrodes each of which can perform electric charge/discharge to corresponding one of the diaphragms independently of the other electrodes, and at least one of the plurality of electrodes is electrically connected with the other electrodes formed for the other diaphragms, and wherein the method includes the step of performing electric charge/discharge between the respective electrodes of the opposed electrodes and the diaphragms appropriately so as to eject ink droplets from the ink chambers. In the present invention, a plurality of electrodes of an opposed electrode are driven in a desired combination, so that the quantity of ink ejected from an ink nozzle (density) can be adjusted in multiple stages. In addition, as an auxiliary operation, for example, ink in the ink nozzles can be vibrated, or the effect of separating ink droplets from the ink nozzles can be enhanced.
(16) In the ink jet head driving method according to the present invention as stated in paragraph (15), each of the opposed electrodes includes a main electrode to be electrically charged and discharged selectively in accordance with a printing pattern, and an sub-electrode formed on the ink nozzle side and electrically connected with other sub-electrodes formed for the other diaphragms. This method includes the step of performing electric charge/discharge between the main electrodes and the diaphragms so that ink droplets are ejected from the ink nozzles, and the step of performing electric charge/discharge between the sub-electrode and the diaphragms so that ink in the ink nozzles is vibrated.
In the present invention, the auxiliary electric charge is performed between the auxiliary electrodes and the diaphragms so that parts of the diaphragms are bent toward the sub-electrodes. Thus, menisci or ink of the ink nozzles can be vibrated without ejecting unnecessary ink droplets. As a result, the menisci can be prevented from being filmed with ink, without ejecting ink droplets. In addition, ink in the ink channels is diffused so that the increase in viscosity of the ink caused by the evaporation of the solvent of the ink can be avoided. In addition, if the sub-electrodes are driven prior to the ejection of ink droplets, a trouble in printing caused by a failure or abnormality in ink ejection can be prevented without consuming ink playing no part in printing, even after no ink droplets has been ejected for a certain time because of no operation of the ink nozzles.
(17) In the ink jet head driving method according to the present invention as stated in paragraph (15), each of the opposed electrodes includes a main electrode to be electrically charged and discharged selectively in accordance with a printing pattern and a sub-electrode formed on the ink nozzle side and electrically connected with other sub-electrodes formed for the other diaphragms. The method includes the step of performing electric charge/discharge between the main electrodes and the diaphragms so as to eject ink droplets from the ink nozzles, and the step of performing electric charge/discharge between the sub-electrodes and the diaphragms so that the ink droplets ejected from the ink nozzles are separated from ink remaining in the ink chambers.
In the present invention, auxiliary electric charge is performed between the sub-electrodes and the diaphragms so that parts of the diaphragms are bent toward the sub-electrodes. As a result, the time for tail portions of ejected ink columns to leave the ink nozzles is shortened so that the effect of separating ink droplets from the ink nozzles can be enhanced. In addition, the menisci in the ink nozzles are drawn into the ink chambers on ejecting ink droplets, so that unnecessary ink droplets can be prevented from being ejected immediately after ejecting ink droplets contributing to printing. Thus, if the sub-electrodes are driven at a predetermined interval after the time when the main electrodes have been driven to eject ink droplets, unnecessary ink droplets can be prevented from being ejected after ejecting the necessary ink droplets. Thus, troubles of printing caused by a failure or abnormality in ink ejection can be prevented, even if ink droplets have been continuously ejected from the nozzles for a long time without wiping the nozzle surfaces.
(18) In the ink jet head driving method according to the present invention as stated in paragraph (15), in the step of performing electric charge/discharge between the main electrodes and the diaphragms to eject ink droplets from the ink nozzles, ink droplets ejected previously are separated from ink remaining in the ink chambers when succeeding ink droplets are ejected immediately thereafter. For example, in the case where one dot is formed of a plurality of ink droplets, the operation described in the paragraph (17) can be obtained by ejecting a following ink droplet.
(19) In the ink jet head driving method according to the present invention as stated in paragraph (15), the main electrodes are provided correspondingly to the diaphragms, and the sub-electrodes include a first sub-electrode provided in common for a plurality of the diaphragms so as to face the diaphragms on the ink nozzle side, and one or a plurality of second sub-electrodes provided in common for a plurality of the diaphragms so as to be disposed between the main electrodes and the first sub-electrode, and wherein the main electrodes and the sub-electrodes are driven in a desired combination so that ink droplets are ejected from the ink nozzles. In the present invention, the main electrodes and the sub-electrodes are driven in a desired combination so that the ink discharge quantity (density) can be adjusted in multiple stages.
(20) In the ink jet head according to the present invention as stated in paragraph (6), the metal is formed of chromium, titanium, aluminum, or platinum.
(21) In the ink jet head according to the present invention as stated in paragraph (2), each of the main and sub-electrodes includes an opposed portion formed of ITO and oppositely disposed to said diaphragm, and a lead portion electrically connected with said opposed portion, and at least the lead portion of the sub-electrode comprises a metal thin film and an ITO thin film formed on the metal thin film.
(22) In the ink jet head according to the present invention as stated in paragraph (21), the metal thin film is formed of chromium, titanium, silver, or an alloy composed of silver, palladium and copper.